Absolutely there are very big differences. The ideology is like night and day between the two parties. I am not going to try and sway you as to what party I believe is right or wrong but there are clear differences. If you would like to know more about the parties you can check out my hub on this subject called "Are You In The Right Political Party". It describes both parties and what they stand for. It could help you determine what party you most likely fit into. No candidate in any party will fit your beliefs 100% but you can get close to most of your beliefs and that will determine what party you should be registered with. From there you can break it down to who you vote for by listening to each candidate in the debates.
"Your second question, Who chooses who runs anyway? Not us..." Initially, each person that runs determines if they are going to run, nobody else determines if someone will run or not. The Primary elections or Caucasus in each state will determine who will be the candidate for their respective party. That is why it is important to vote in the correct party that you most closely match in beliefs or ideology. That way you have your say as to who the candidate for your party will be.
It was a very good question and I am sure many other people were thinking about that for a long time but never asked. Thanks.